A currently accepted technique for assessing the severity of a stenosis in a blood vessel, including ischemia causing lesions, is fractional flow reserve (FFR). FFR is a calculation of the ratio of a distal pressure measurement (taken on the distal side of the stenosis) relative to a proximal pressure measurement (taken on the proximal side of the stenosis). FFR provides an index of stenosis severity that allows determination as to whether the blockage limits blood flow within the vessel to an extent that treatment is required. The normal value of FFR in a healthy vessel is 1.00, while values less than about 0.80 are generally deemed significant and require treatment. Common treatment options include angioplasty and stenting.
Coronary blood flow is unique in that it is affected not only by fluctuations in the pressure arising proximally (as in the aorta) but is also simultaneously affected by fluctuations arising distally in the microcirculation. Accordingly, it is not possible to accurately assess the severity of a coronary stenosis by simply measuring the fall in mean or peak pressure across the stenosis because the distal coronary pressure is not purely a residual of the pressure transmitted from the aortic end of the vessel. As a result, for an effective calculation of FFR within the coronary arteries, it is necessary to reduce the vascular resistance within the vessel. Currently, pharmacological hyperemic agents, such as adenosine, are administered to reduce and stabilize the resistance within the coronary arteries. These potent vasodilator agents reduce the dramatic fluctuation in resistance (predominantly by reducing the microcirculation resistance associated with the systolic portion of the heart cycle) to obtain a relatively stable and minimal resistance value.
However, the administration of hyperemic agents is not always possible or advisable. First, the clinical effort of administering hyperemic agents can be significant. In some countries (particularly the United States), hyperemic agents such as adenosine are expensive, and time consuming to obtain when delivered intravenously (IV). In that regard, IV-delivered adenosine is generally mixed on a case-by-case basis in the hospital pharmacy. It can take a significant amount of time and effort to get the adenosine prepared and delivered to the operating area. These logistic hurdles can impact a physician's decision to use FFR. Second, some patients have contraindications to the use of hyperemic agents such as asthma, severe COPD, hypotension, bradycardia, low cardiac ejection fraction, recent myocardial infarction, and/or other factors that prevent the administration of hyperemic agents. Third, many patients find the administration of hyperemic agents to be uncomfortable, which is only compounded by the fact that the hyperemic agent may need to be applied multiple times during the course of a procedure to obtain FFR measurements. Fourth, the administration of a hyperemic agent may also require central venous access (e.g., a central venous sheath) that might otherwise be avoided. Finally, not all patients respond as expected to hyperemic agents and, in some instances, it is difficult to identify these patients before administration of the hyperemic agent.
To obtain FFR measurements, one or more ultra-miniature sensors placed on the distal portion of a flexible device, such as a catheter or guide wire used for catheterization procedures, are utilized to obtain the distal pressure measurement, while a sensor connected to a measurement instrument, often called the hemodynamic system, is utilized to obtain the proximal or aortic pressure measurement. Currently only large expensive systems or a combination of multiple devices connected to the distal pressure wire and the hemodynamic system can calculate and display an FFR measurement. In that regard, to calculate the FFR these devices require both the aortic or proximal pressure measurement and the coronary artery or distal pressure measurement. Accordingly, these systems require the catheter lab's hemodynamic system to have a high level analog voltage output. “High level” in this context generally implies 100 mmHg/Volt output. Unfortunately, there are many hemodynamic systems that don't provide a high level output. As a result, when using these hemodynamic systems, providing an FFR measurement is difficult if not impossible. Further, space in a typical catheter lab is extremely limited. Consequently, devices that are large and located in the catheter lab are disfavored compared to smaller derives, especially if the smaller device can provide much if not all of the functionality of the larger device. As a result, it is highly desirable to have a device that that can display FFR and yet is small and lightweight that can sit on, or near, the patient bed and be easily read by the physician.
Further, most pressure measurement devices require an extra source of power like an AC adapter or wall plug. This adds to wire clutter and available medical grade AC outlets are not often available near the patient bed. In addition, any device that uses AC power must undergo stringent safety precautions to reduce patient risk due to leakage currents. Batteries are another alternative for power. But, batteries must be replaced, disposed of correctly and have a finite shelf life.
Accordingly, there remains a need for improved devices, systems, and methods for assessing the severity of a blockage in a vessel and, in particular, a stenosis in a blood vessel. In that regard, there remains a need for improved devices, systems, and methods for providing FFR measurements that have a small, compact size (e.g., suitable for hand-held use), integrate with existing proximal and distal pressure measurement devices, and do not require a separate power source.